During both offshore and land-based oil and gas drilling operations, the controlled containment and diversion of wellbore fluids and gas returns at the wellhead assembly presents a significant challenge. Gases dissolved in the wellbore fluid may rapidly decompress and expand while ascending the wellbore. Upon reaching the wellhead assembly, the wellbore gases may produce a shock known in industry as a “kick”. Flow surges from the hydrocarbon producing formation can also result in shock waves in the wellbore fluid and kicks at the wellhead assembly. Kicks can be anticipated by detecting gas entry into the wellbore and significant changes in the wellbore fluid flow rate. Even if anticipated, however, kicks may subject the wellhead assembly to extreme and sudden pressure increases that can damage rig equipment or result in spillage and venting of wellbore fluids and gases. These undesirable effects can threaten the safety of rig operators and contaminate the environment.
In offshore drilling operations, the wellbore fluids are conveyed from the seafloor to a wellhead assembly on a floating drill ship or a drilling platform within a riser, the riser comprising, a conduit formed by lengths of pipe attached by flanged connections. Typically, a riser diverter is positioned at the head of the riser in series with a blowout preventer. The riser diverter has outlet and vent lines to direct wellbore fluid and gas returns away from the well head and the drilling platform. The blowout preventer has hydraulically and remotely actuated valves. In the event that the drilling crew loses pressure control over the wellbore fluid, the valves of the blowout preventer are actuated to close and halt the flow of wellbore fluid in the riser.
In conventional land-based oil and gas drilling operations, the wellbore fluids are conveyed from the wellbore to the wellhead assembly on the surface within a casing string. A top stack having a blowout preventer may be positioned at the top of the wellhead assembly. The blowout preventer may be of the ram type having gate-like or valve-like elements or the annular type having elastomeric sealing elements, which are mechanically actuated to constrict or close off the flow of wellbore fluid in the casing string.
Although these conventional wellbore fluid control devices provide some protection against kicks, it would be advantageous to have an additional pressure barrier between the wellbore fluids and the external environment for use in both off-shore and land based drilling operations. It would also be advantageous if such secondary pressure barrier could be relatively simple and easily installed on a conventional riser diverter assembly or on a blowout preventer stacks annular.